What might it be about a subterranean art commission which makes the imagination soar? Michael Landy, Jeremy Deller and Eva Rothschild are among the well-known contemporary artists to have taken their talents underground in recent times. It's a gallery space you probably know, and chances are you have travelled on it.

Art on the Underground is the long running public art project which brings the best art available to the platforms, stations and trains of the world’s oldest subway. And Director Tamsin Dillon reckons this tough brief is also an inspiring one for artists.

"They're interested in pushing themselves," she tells me via phone. "And pushing their practice in a way that's beyond what they might experience making work in a gallery situation."

Dillon explains that more than three million people use the tube every day, and admits that not all of those have an interest in contemporary art. However, artists do have an interest in them.

"Many of the artists we've been working with have been very excited by the idea that the work that they’re presenting is going to be seen by a huge and diverse audience," she says.

Another challenge is the mass transit system running through the 270 "spaces" on the network. "There are huge constraints and obviously the operation of the railway has to take priority.

"There's the difficulty of how you as an artist might engage with a station building that's got a very specific design to it. But also it's filled with advertising, it's got lots of signage, it's got lots of things that are going to distract from an artwork - so it's also how they will deal with that."

Perhaps no less of an impediment to the creation of new ideas is the extensive art and design heritage of the London Undergrdound. Typographer Edward Johnston and map designer Harry Beck are hard acts to follow.

"It has definitely, over almost a century, built up a reputation for excellent world class design in terms of its architecture, the design of the tube map, obviously, and its font, so it's all of its graphic output."

The tube has also long worked with artists on posters promoting tube use and the exploration of London. And yet the remit of Art on the Underground is more than promoting Oyster card top ups.

"We want the programme there to enhance the journeys of the people," says Dillon. "But instead of being decoration, it's important for the programme to reflect London and reflect how important London is for contemporary art."

Even the front of the tube map, a seemingly straightforward canvas, is not without its challenges and rewards. It's "really a travel tool - they are a very integral part of what people use to navigate the system and the network," says Dillon

"Now this series has become something which people expect to see both within the organisation and outside of it."

But having established the map covers, wrapped whole trains, clad stations, posted cross-track posters and introduced film to its repertoire, Art on the Underground has a well-earned track record.